1. Field of the Invention
The present invention regards a data communication system of the so-called field bus type, i.e. having a long twin lead cable to which are connected a number of electronic units which may communicate in that they are supplied with current or voltage from the bus cable and furthermore use this for the data transmission by means of pulse techniques. Such systems would find wide application, e.g. for process control in factories, where signalling may take place selectively directly between the connected units via the bus cable, whereby individual cables between the units and the central control units may be avoided to a large extent.
2. Description of the Prior Art
However the systems developed to date have a number of disavantages. In a system of moderate size the lenght of the bus cable may well be in the order of one kilometer, while a conventional centrally connected system may have a much larger cable consumption with the large installation costs it entails.
The centrally connected systems have the advantage that the connection between the control unit and the individual units "in the field" may be provided in an individually adjusted manner, and generally individual connections will be shorter and many very much shorter than the corresponding bus cable. This relatively greater length of a bus cable has given rise to noticeable problems in the known field bus cable systems. Hence there may in particular in long cables be noticeable disturbances caused by common mode inteference, i.e. where a current is induced in the cable because of external influences which has the same magnetic field direction in both the conductors in the cable. Those units which are receptive in the system continuously check the cable for signals with a special address to the individual units, and in the case of error signals serious errors may occur, both regarding the address and the content of the subsequent information signal. It is known that a transformer may be used at each unit for suppressing these error signals, viz. a balanced transformer with two primary coils in series regarding high frequencies which normally cooperate but which in case of common mode interference will give rise to oppositely directed magnetic fields from the two coils whereby the error signal is suppressed. The merthod may be efficient.